Basic performance properties necessary for light-emitting device substrates include high reflectivity, high heat dissipation capability, high dielectric voltage, and long-term reliability.
Representative examples of such light-emitting device substrates include those using ceramic substrates. Such a light-emitting device substrate is produced by forming an electrode pattern on a plate-shaped ceramic substrate. With the trend toward high power light-emitting devices, an improvement in brightness is achieved by arranging a large number of light-emitting elements. This has led to a steady increase in size of ceramic substrates year by year.
Specifically, when a general LED light-emitting device used with an input power of 30 W is realized by, for example, arranging blue LED elements with dimensions of around 650 μm×650 μm on one substrate classified as a medium size substrate, about 100 LED elements are necessary. One example of the ceramic substrate on which about 100 LED elements are arranged is a ceramic substrate having a thickness of about 1 mm and a plane size of 20 mm×20 mm or larger.
To realize a brighter light-emitting device for LED lighting with an input power of 100 W or more on the basis of technical development regarding upsizing of substrates, there is a need for a larger-size ceramic substrate with a plane size of 40 mm×40 mm or larger on which 400 or more LED elements can be placed at once.
However, implementation of large-size ceramic substrates on a commercial basis in response to the need for the large-size ceramic substrates described above has been difficult because of the following three issues: the strength, production accuracy, and production cost of the substrates.
A ceramic material is basically pottery, and one problem with a large ceramic substrate is its strength. When the substrate is increased in thickness in order to overcome the above problem, its thermal resistance becomes high (its heat dissipation capability deteriorates), and a new problem arises in that the material cost of the substrate increases. When a ceramic substrate is increased in size, not only the outer dimensions of the substrate but also the dimensions of an electrode pattern formed on the substrate tend to lose their accuracy. This results in a problem in that a reduction in production yield and an increase in production cost of the substrate tend to occur.
To overcome the foregoing problems with upsizing of ceramic substrates, PTL 1 to PTL 4, for example, propose light-emitting device substrates in which a ceramic-based paint is used to form a light reflecting layer on a surface of a metal base. These light-emitting device substrates have excellent reflectivity and dielectric voltage, and therefore good light-emitting device substrates can be realized.